1. Field of the Invention
When a fault occurs in an electrical power supply network, an electrical travelling wave will propagate in the network and, because of repeated reflections in various discontinuities in the network, such as transformers, stations and the source of the fault, currents and voltages will contain transients which are inconvenient for measuring relays in the relay protection devices. The frequencies dominating in these transients depend on the distance to the fault and the network configuration. Investigations have shown that, even with the introduction of suppression filters for the transients, it will be difficult to use, for example, conventional impedance relays for relay protection devices, the operating time of which is to be less than the time of a cycle. In case of higher system voltages, higher demands are placed on the rapidity of the relay protection devices, while at the same time the damping of the transient waves decreases. The application of traditional measuring principles for the relay protection devices will therefore be more difficult to carry out.